This invention relates generally to flame retardant polyolefins, and more particularly concerns a method for mixing flame retardant compounds with polypropylene resins prior to extrusion and spinning to produce extruded fine denier filaments or fibers which are highly flame retardant and which exhibit good heat and color stability.
Widespread use of plastics in products for industry, commerce, and the household, has created the need to assure that such plastics are flame retardant. Polyolefins, for example, are highly flammable, and when used in the manufacture of fibers for clothing, upholstery, carpeting, and the like, the need for flame retardancy is even more apparent. It is, likewise, well recognized that polyolefin fibers used in upholstery and carpets have in the past increased the severity of fires in large commercial establishments by generating explosive gases. When the gases are ignited, the explosion can cause the fire to burst outside of the building and then chain link along the outside of the building from floor to floor.
As a result of the potential fire hazard created by the use of polyolefin fibers in materials, various governmental agencies and industrial societies have developed tests for determining the fire retardancy of materials made from various fibers. Five tests which are well recognized in the industry include:
(1) Motor Vehicle Interiors--MUSS-302;
(2) FAA Vertical Flammability, 14 C.F.R. 25-853(B);
(3) UFAC Cigarette Test Class II;
(4) Drapery and Curtain Fabrics--NFPA 701; and
(5) Tunnel Test--ASTME 84 (carpet).
In order to render materials containing polyolefin fibers flame retardant various fiber additives and finishing treatments have been proposed. With respect to additives, Hancock U.S. Pat. No. 4,532,278 discloses a fire retardant polypropylene fiber which results from combining the polypropylene polymer with a bromophenyl compound and which may include C.sub.12 H.sub.2 Br.sub.8 O; tetrabromobisphenol A; bis(2-hydroxyethyl ether) of tetrabromobisphenol A; and octabromobisphenyl oxide. The flame retarding compound, according to the Hancock, is simply blended into the polypropylene resin prior to or during extrusion.
Saiki et al. U.S. Pat. No. 4,273,899 discloses a process for forming a fire retardant thermoplastic polymer, specifically polyester, which process may involve multiple pelletizing steps in order to blend the fire retarding compound with the thermoplastic polymer. The resulting polyester is used in making injection-molded parts. The flame retardant compound is a medium molecular weight polymer of a carbonate of a brominated dihydric phenol having at least 16 recurring units optionally with an antimony compound.
While the prior art has addressed the problem of providing flame retardant polyolefin fibers and materials by the use of flame retarding additives, the prior art has not specifically addressed the particular problems which exist in rendering fine denier filaments of polypropylene flame retardant. Fine denier polypropylene fibers are manufactured by extruding the melted polypropylene resin through very fine nozzles or orifices of a spinneret. If the flame retardant compound is not completely mixed with the melted polypropylene resin prior to extrusion, the fire retardant compound tends to agglomerate onto itself, creating lumps in the extrusion melt which will clog the spinneret filters or orifices and will result in down time on a commercial line. In addition, smaller lumps of the flame retardant compound which can pass through the filter and spinneret die orifice will displace sufficient polymer in the resulting fine denier filament to create a weak spot in the continuous filament which may result in breakage that shuts down the production line. In addition, incomplete mixing of the flame retardant compound with the polypropylene resin may also have a deleterious affect on the ability of the flame retardant compound to generate cooling gases within a localized area which serve to render the material flame retardant.